This invention relates to a backing device for use with piezoelectric crystals, the backing device being a multiphase material having high attenuation characteristics with an impedance closely matched to that of the crystal.
The most effective method of generating and receiving ultrasonic waves is using piezoelectric crystals. An electric impulse applied to such a crystal excites a relatively long duration acoustic-pulse due to the crystals relatively low damping coefficient, namely, a high-Q. For nondestructive evaluation (NDE) applications, such as depth resolution and defect characterization, there is a need for acoustic pulses of as short as possible duration. To reduce the pulse duration a backing material, with an impedance closely matched to the crystal, should be used. For practical purposes, that is, for obtaining a transducer of a small size, the backing material must have as high attenuation as possible to eliminate back reflections.
In the prior art, it is a common practice to use a two-phase mixture consisting of a matrix and a powder filler. See the following references: V. M. Merkulova, "Acoustical Properties of Some Solid Hetergeneous Media at Ultrasonic Frequencies," Sov. Phys - Acoustics 11, (1) 1965; P. J. Torvik, "Note on the Speed of Sound in Two Phase Mixtures," J. Acoust. Soc. of Amer. 48, (2) 1970; S. Rokhlin, S. Golun and Y. Gefen, "Acoustic Properties of Tungsten-Tin Composites," J. Acous. Soc. of Amer. 69 (5) 1981; V. V. Sazhin, F. I. Isaenko and V. A. Konstantinov, "Mechanical Damper for Ultrasonic Probes," Sov. J. of NDT. 9 (5) p. 505-607 (1973); J. D. Larson and J. G. Leach, "Tungsten-Polyvinyl Chloride Composite Materials - Fabrication and Performance"; and, S. Lees, R. S. Gilmore and P. R. Kranz, "Acoustic Properties of Tungsten-Vinyl Composites," IEEE Trans. on Socis and Ultrasonics, SU-20 (1), 1973.
The matrix usually has a high absorption coefficient, the filler induces strong scattering and combined they provide the required high attenuation. The proper selection of materials and volume fractions allows matching impedances to the crystal.
Tungsten/epoxy is the most widely used backing for commercial transducers due to its potential in providing a large range of impedances (Z) between 3 and 100.times.10.sup.5 g/cm.sup.2 sec. and its sufficiently high attentuation. The characteristic curve of Impedance vs. Volume Fraction, shown in FIG. 1, shows a very slow increase in impedance for increasing volume fraction of tungsten up to about 0.8, above which a sharp increase occurs. Matching the impedances of crystals such as PZT and LiNbO.sub.3, with an impedance of about 30 to 35.times.10.sup.5 g/cm.sup.2 sec., requires a high volume fraction of tungsten, but this is subject to physical packing limits. Moreover, the steep slope in this range makes reproducibility of backing impedance difficult to obtain. These obstacles are common to all two-phase combinations which serve as potential backing materials.